The invention relates to a lighting device for a motor vehicle.
Nowadays, LED lighting devices are increasingly used in motor vehicles, by which suitable light distributions are generated for signal lights or headlights of the vehicle. Since the luminance for LEDs is relatively low, large installation spaces are required for the lighting device, particularly for generating light for high ranges. Lighting devices based on LEDs are not efficient in smaller installation spaces. In addition, it is difficult to generate a high-precision arbitrarily shaped light distribution by the use of LEDs.
It is an object of the invention to create a lighting device for a motor vehicle by which highly precise or efficient light distributions can be generated in small installation spaces.
This and other objects are achieved by a lighting device according to the invention for a motor vehicle, particularly for a passenger car and, if necessary, also for a truck. The lighting device includes one or more lighting units, a respective lighting unit being further developed such that, when it is operated by a laser light, it generates a punctiform light source. In this case, a punctiform light source is a light source having a small light-emitting surface and a large light flux, i.e. high luminance, which, with respect to the dimensions of the lighting device, in good approximation, may be assumed to be punctiform in that all rays of the light source originate from a single point.
The lighting device according to the invention further includes an optical device, which is further developed and arranged with respect to the lighting unit or lighting units such that a specified light distribution from the light of the punctiform light source will be generated after passing through the optical device. In this case, the optical device may, if required, be constructed of several partial units which, each separately, are assigned to a lighting unit. Likewise, there is the possibility that the optical device represents a continuous unit, on which the light of all lighting units is incident.
The lighting device according to the invention has the advantage that a punctiform light source with a very high luminance can be generated in a simple manner by way of laser light, so that the corresponding dimensions of the lighting device, particularly of the optical device for generating the specified light distribution, can be selected to be compact. In a particularly preferred embodiment, the maximal dimension of the punctiform light source in a top view, i.e. in the main radiation direction with the greatest intensity of the light source, amounts to 500 μm or less, preferably 100 μm or less, and particularly preferably 20 μm or less. Furthermore, in the top view, the punctiform light source preferably has a emitting surface of 0.5 mm2 or less, particularly of 0.01 mm2 or less, and particularly preferably of 0.0002 or less. The punctiform light source includes particularly an emitting cornered surface whose edges have a length of 500 μm or less and preferably 20 μm or less. The punctiform light source with the above-described dimensions is preferably further developed such that it generates a light flux of 100 lm or more and particularly of 200 lm or more and/or has a radiant power of 1 Watt or more and/or a luminance of at least 105 Cd/m2 particularly of 109 Cd/m2 or more. Such punctiform light sources can only be obtained by way of laser light, for example, by using laser diodes.
As a result of the use of a punctiform light source, light distributions freely shaped by the optical device connected on the output side can be generated without a problem depending on the usage purpose of the lighting device. In particular, the shape of the optical device for generating a defined light distribution can thereby be determined in an algorithmically particularly simple manner, because the corresponding calculations can be based on an ideal punctiform light source. For calculating the shape of the optical device, particularly the so-called ITWM algorithm can be used which was developed by the Fraunhofer Institut für Techno- and Wirtschaftsmathematik.
In a further development of the lighting device according to the invention, at least one lighting unit generates a punctiform white light source during the operation. This variant will be used particularly when the lighting device is to take over the function of a vehicle headlight.
In a further particularly preferred embodiment of the invention, at least one lighting unit includes one or more laser diodes, particularly one or more monochromatic laser diodes. Nowadays, such laser diodes have radiances so that punctiform light sources can be generated that have a high luminance. In this case, the direct radiance of laser diodes is significantly higher than that of conventional LEDs. The radiance of a blue laser diode currently amounts to approximately 7×105 Watts/cm2, while the radiance of an LED is only at 20 Watts/cm2. Preferably, laser diodes with a luminous power of at least 1 Watt and particularly between 1.5 and 3.0 Watts or between 1.5 and 5 Watts are used in the lighting device according to the invention.
In a further preferred embodiment, at east one lighting unit has a converter which generates white light from the monochromatic light of the laser diode or laser diodes. In this case, the converter may be a phosphor conversion layer, on which the laser radiation of the laser diode or diodes is focused by use of optics (for example, a lens), whereby the punctiform white light source is generated on the phosphor conversion layer. Phosphor conversion layers for converting monochromatic light are known per se. For example, in the case of a blue laser diode with an emission wavelength of 450 nm, a phosphor conversion layer of Ce:YAG phosphor can be used for generating white light. For violet laser light with a wavelength of 405 nm, particularly a phosphor conversion layer of cerium-doped nitride phosphor or cerium-doped oxynitride phosphor is used.
For the conversion of monochromatic laser light to white light, as required, also an optical waveguide can be used, in which the monochromatic laser radiation of several monochromatic laser diodes may be appropriately combined for generating white light. The punctiform white light source may be formed at the exit surface of the optical waveguide. Likewise, optics ay be provided which appropriately focus the white light exiting from the optical waveguide for forming a punctiform light source.
In a further development of the invention, at least one lighting unit has a single monochromatic laser diode which represents a punctiform light source. This variant will be used particularly when a light of a certain color is to be generated by means of the lighting device.
Depending on the application case, the number of lighting units built into the lighting device may vary. In a preferred embodiment, the lighting device has 10 or less lighting units, particularly 3 to 6 lighting units.
Depending on the application case, the lighting device according to the invention can take over various functionalities. In one embodiment, the lighting device is a vehicle headlight. A vehicle headlight is distinguished in that it actively illuminates the environment of the vehicle. As required, the lighting device according to the invention may also be a vehicle signal light, which is distinguished in that it is used only for signaling to other traffic participants. In a preferred embodiment, the lighting device is further developed such that it can generate a low-beam light characteristic as a specified light distribution during operation. The low-beam light characteristic is known per se and is also specified in legal norms or standards (for example, SAE Standards). A low-beam light is distinguished in that the angular distribution of the radiated light has a horizontal light-dark cutoff in order not to blind oncoming traffic participants. As a rule, the angular distribution for illuminating the side of the road has a wider layout. A low-beam light further has a finite range, which is approximately at 65 m. By means of the lighting device according to the invention, it becomes possible to generate a light-dark cutoff with a high contrast for a low-beam light.
In a further embodiment of the invention, the lighting device s further developed such that, during operation, it can generate a high-beam light characteristic during operation as a specified light distribution. The light distribution of high-beam light is known per se and is distinguished in that the light has a greater range than low-beam light and is therefore concentrated in a smaller angular area around the vehicle longitudinal axis. In particular, no range limitation is specified for high-beam light. While the light values remain the same, a reduction of the light exit surface generally results in increase of glare values to which an oncoming mild be exposed. In the case of the high-beam light function, a size reduction and thus an increase of the glare values would not be a problem because the operation of high-beam light is, as a rule, intended only when there is no oncoming traffic. Nonetheless, by means of the lighting device according to the invention, as required, a glare-free high-beam light can also be implemented which has a light distribution with a vertical bright-dark cutoff with a contrast that is as high as possible with respect to the shadow area, in which the oncoming traffic is situated. In this case, the punctiform light source allows a significantly more precise implementation of the bright-dark cutoff and therefore reduces the glare effect also in this case.
In a further preferred embodiment, one or more lighting units include, in addition to the punctiform light source generated by laser light, one or more conventional LEDs or light emitting diodes, whose generated light is not laser light. In this case, the lighting device is further developed such that it can generate a light characteristic which is composed of the light of the LED or LEDs and the light of the punctiform light source. The punctiform light source preferably generates that fraction of the light characteristic which requires particularly high precision. In particular, the light characteristic may again be a high-beam light characteristic, the punctiform light source being responsible for that fraction of the high-beam characteristic which represents the central high-beam light spot for reaching the range of the high-beam light. The light distribution around this high-beam light spot with the lower range is, in this case, generated by the LED or LEDs. If necessary, there is also the possibility that the LED or LEDs and the punctiform light source are switched separately for generating certain light distributions.
The lighting device according to the invention may also be provided for generating other light distributions. The lighting device may particularly also take over the function of several different headlights or signal lights. In particular, the lighting device may also be a daytime running light, an indicator light, a backup light, a fog light, a turn signal light, a marker light, a taillight, a brake light and/or a dynamic spot light or the like.
In a further preferred embodiment, the lighting device is further developed such that, during operation, it can generate one or more specified structures, particularly one or more laminar shapes, one or more strips, one or more spots, one or more arrows, one or more symbols and/or one or more signs, as specified light distributions.
The optical device used in the lighting device according to the invention be further developed differently depending on its usage. This device preferably comprises at least one reflector and/or at least one lens.
As mentioned above, because of the use of punctiform light sources with a high luminance, the lighting device may have compact dimensions. In a preferred embodiment, the maximal dimension of the optical device in the top view (i.e. viewed in the main radiation direction with the highest intensity), is 50 mm or less, particularly 30 mm or less.
In addition to the above-described lighting device, the invention also includes a motor vehicle, which has one or more of the lighting devices according to the invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.